Power Delivery on USB-C means that the charger by default works at 5V, delivering up to about 2.4A (12W total), like smaller USB chargers. But PD allows a device to ask for more voltage, which increases the wattage. Your MacBook for example, asks for ~15V @ 2A for charging (30W). My MacBook Pro asks for 20V @ 3A for charging from it's 60W adapter.
The iPad Pro does seem to be fine asking for up to 30W or so. Using 9V or 15V, whichever is available, maxing out at 3A (so 27W if only 9V is available, or 12-15W when only 5V is available).
I do agree with tekchic's comment about cheap third party USB-C chargers. But generally the problem is that so many get stuck at 5V, and won't actually fast charge your iPad. At least this was true with the 10.5" iPad Pro.